1. Field of the Invention
The present invention relates to ink jet printing systems in general, and more particularly, to maintenance devices used to improve the performance of said printers.
2. Prior Art
The use of nonimpact printers using multinozzle or single nozzle print heads for printing readable data on a recording surface is well known in the prior art. Such printers may be divided into the drop-on-demand type printers and the continuous type printers. In the drop-on-demand type printers, a drop of print fluid is generated from the print head or the drop generator when needed. In the continuous type printers, continuous streams of ink are extruded from the drop generators. A vibrating crystal vibrates the ink so that the continuous streams are broken up into regularly spaced constant size droplets. The droplets are used selectively for printing on the recording surface. Although the present invention finds use with either type of ink jet printer, it works well with the continuous type printers and therefore will be described in association therewith.
The prior art abounds with continuous type ink jet printers. Generally, these printers consist of a print head. The print head generates the ink droplets which are used to write on the recording media. The print head consists of a fluid chamber in which ink (which may be magnetic or conductive) is forced in under pressure. One or more discharging nozzles are in fluidic communication with the pressurized ink. A vibrating crystal in the fluid chamber perturbs the ink so that fluid emanating from the nozzles is broken up into droplets. The droplets are subsequently influenced by electric or magnetic means whereby some are used to print data onto a recording surface. Ink droplets which are not needed for printing are collected by a gutter assembly and returned or recirculated to the ink supply system for reuse. U.S. Pat. Nos. 3,848,118 and 3,924,974 are examples of this prior art.
One of the problems which plagues this prior art ink jet system is the inability to control the streams so that ink jet components such as charge electrodes and deflection electrodes are not contaminated with the ink. The problem is particularly pronounced at start-up and/or shutdown of the system. During the start-up and/or shutdown interval, the behavior of the streams tend to be nonstable or erratic and, as a result, wetting of the components is inevitable.
It is believed that the stream's erratic behavior or stream's misdirectionality is caused by one or more of the following factors: (a) the presence of foreign material in the ink jet head, (b) lack of control over ink movement in the head, (c) the presence of air in the head and (d) relatively high compliance of the ink jet head.
Foreign materials that are trapped inside an ink jet head have the potential to obstruct ink flow through the nozzles and seriously degrade head reliability. Generally microscopic size particles pass through the nozzles. These particles are likely to change the stream's break-off characteristics and affect the trajectory of the droplets. Large size particles and gas bubbles are more disruptive to the ink flow. Large size particles may be solids or nonsolids. The solid particles tend to partially close the nozzle openings. The partial closing reduces ink flow through the nozzle. The result is that the ink stream break-off distance is shortened which further results in stream misdirectionality. Nonsolid particles tend to form globules that seal off the nozzles and stop ink flow. Gas bubbles tend to seal off the nozzles and stop ink flow. Over a period of time, these bubbles partially dissolve until they are small enough to pass through the nozzles. As they exit the nozzles, they explode, causing splatter on nearby objects.
The gas bubbles also act as shock absorbers. They compress as ink pressure increases and expand as the pressure decreases. This increases the compliance characteristics of the ink jet head.
The need to control ink movement through the head of an ink jet printer is critical at start-up and/or shutdown. During normal operation, the head is pressurized. At shutdown, the head pressure goes from a positive value to ambient or subambient value. Ideally, the change in pressure should be instantaneous with no overshoot. However, to depressurize the head requires the removal of ink. When nozzles are used to vent ink from the head, the head's pressure decays exponentially. One of the adverse effects of exponential decay is that the streams usually vary from the normal trajectory. As the streams vary, gravity becomes the dominant force acting on the streams. Since gravity tends to pull an object downwardly, the ink generally oozes from the nozzles and wets the components below. The longer the decay, the greater the problem. At start-up or turn-on time, the ideal condition is for the pressure in the head to rise instantaneously from ambient to a positive operating value. However, each ink jet head has its own characteristic compliance which forces the pressure to rise exponentially. As with exponential pressure fall, exponential pressure rise results in stream instability and subsequent component's contamination. The longer the rise time, the more pronounced the contamination.
The presence of air in the ink jet head is another factor which degrades the performance of the head. The air forms bubbles which act as shock absorbers. These shock absorbers degrade the compliance of the head. Compliance refers to the response time for the head. It is the time which is needed to turn the head on or off. The head is turned on when the streams are properly oriented and can be used for writing on a support media. The shorter the time, needed to turn on or turn off the head is, the better the head's compliance is. It therefore behooves the user to exclude air from the head.
Air may enter the head due to a phenomenon referred to as thermal cycling. Thermal cycling is the term used to describe the temperature fluctuations associated with a head. The temperature fluctuation changes the volume of ink in the head. When the temperature decreases, the ink volume contracts and air is drawn into the head. When the temperature increases, the ink volume increases and the excess ink dribbles from the nozzle to contaminate adjoining components.
U.S. Pat. No. 3,805,276 describes a device for removing air from an ink jet recording apparatus. The device includes a supplementary ink holder and a valve.
The input end of the valve is coupled to the conduit which supplies ink to the nozzle. The output end of the valve is coupled to the ink holder. During nonprinting periods, the valve is opened and air escapes from the conduit into the tank.